1. Field of the Invention
This invention relates to a stable charge transport layer composition of a photoreceptor used in electrophotography and a method of making the same. More in particular, the invention relates to a specific formulation for a charge transport layer containing both polytetrafluoroethylene (PTFE) particles and hydrophobic silica particles, the formulation forming a very stable dispersion for coating and achieving a charge transport layer imparting superior wear resistance to a photoreceptor and improved toner cleaning.
2. Description of Related Art
In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer. The imaging members can take several forms, including flexible belts, rigid drums, etc. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder.
U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having a separate charge generating (photogenerating) layer (CGL) and charge transport layer (CTL). The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. The photogenerating layer utilized in multilayered photoreceptors include, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive materials may be formed as a continuous, homogeneous photogenerating layer.
Examples of photosensitive members having at least two electrically operative layers including a charge generating layer and diamine containing transport layer are disclosed in U.S. Pat. Nos. 4,265,990, 4,233,384, 4,306,008, 4,299,897 and 4,439,507. The disclosures of these patents are incorporated herein in their entirety.
Charge transport layers are known to be comprised of any of several different types of polymer binders that have a charge transport material dispersed therein. However, these conventional charge transport layers suffer from a fast, nearly catastrophic wear rate of 8 to 10 microns or more per 100 kilocycles when the photoreceptor is charged using an AC bias charging roll (BCR). The use of AC bias charging rolls to charge a photoreceptor surface is conventional in the art for forming images in low speed, for example up to 40 ppm, imaging devices (e.g., copiers and printers). However, the corona generated from the AC current, applied to the BCR, decomposes on the top photoreceptor layer. The decomposed material can be easily removed by a cleaning blade. Such a repeated process during the printing cycle wears out the photoreceptor top layer very quickly.
Wear rate is a significant property in that it limits the life of the photoreceptor, and photoreceptor replacement in electrostatographic devices such as copiers and printers is very expensive. It is thus very significant to limit wear of the photoreceptor so as to achieve a long life photoreceptor, particularly with respect to small diameter organic photoreceptor drums typically used in low speed copiers and printers that are charged with an AC BCR. In such small diameter drums, 100 kilocycles translates into as few as 10,000 prints. CTL wear results in a considerable reduction in device sensitivity, which is a major problem in office copiers and printers that typically do not employ exposure control. In addition, the rapid wear of the top photoreceptor layer requires better cleaning of the debris from the photoreceptor surface in order to maintain good toner transfer and good copy quality.
U.S. Pat. No. 5,096,795, incorporated herein by reference in its entirety, describes an electrophotographic imaging member comprising a charge transport layer comprised of a thermoplastic film forming binder, aromatic amine charge transport molecules and a homogeneous dispersion of at least one of organic and inorganic particles having a particle diameter less than about 4.5 micrometers, the particles comprising a material selected from the group consisting of microcrystalline silica, ground glass, synthetic glass spheres, diamond, corundum, topaz, polytetrafluoroethylene, and waxy polyethylene, wherein said particles do not decrease the optical transmittancy or photoelectric functioning of the layer. The particles provide coefficient of surface contact friction reduction, increased wear resistance, durability against tensile cracking, and improved adhesion of the layers without adversely affecting the optical and electrical properties of the imaging member. Specific materials as in the present invention are not taught, nor is it taught to use the charge transport layer in an apparatus employing an AC bias charging roll.
U.S. Pat. No. 5,725,983, incorporated herein by reference in its entirety, describes an electrophotographic imaging member including a supporting substrate having an electrically conductive layer, a hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, an anticurl back coating, a ground strip layer and an optional overcoating layer, at least one of the charge transport layer, anticurl back coating, ground strip layer and the overcoating layer comprising a blend of inorganic and organic particles homogeneously distributed in a film forming matrix in a weight ratio of between about 3:7 and about 7:3, the inorganic particles and organic particles having a particle diameter less than about 4.5 micrometers. These electrophotographic imaging members may have a flexible belt form or rigid drum configuration. The ability of hydrophobic silica to stabilize a dispersion containing polytetrafluoroethylene particles as in the present invention is not taught, nor is it taught to use the charge transport layer in an apparatus employing an AC bias charging roll.
Thus, it has been broadly known to attempt to utilize small particles such as polytetrafluoroethylene in outer layers of a photoreceptor in an effort to increase the hardness/durability of the outer photoreceptor layers. However, these particles have been difficult to disperse uniformly in the materials typically used for certain layers of the imaging member, particularly the charge transport layer. When a charge transport layer is formed from a dispersion in which such particles are poorly dispersed, the imaging member exhibits lesser electrical performance and poorer print quality. Poor dispersion causes high residual voltage (Vr) and Vr cycle-up, as well as leading to non-uniform coatings that contain large size particle aggregates (since poor dispersion prevents uniform aggregates from forming). The presence of large size aggregates lessens print quality as they cause white spots to occur in a solid image area. The large aggregates on the surface also cause difficulty in toner cleaning during the printing cycles. Poor cleaning can cause non-uniform density, such as streaks, to print-out. Poor cleaning also reduces toner transfer efficiency and increases toner waste.
Moreover, particles such as polytetrafluoroethylene slowly settle over time in a CTL coating dispersion as a result of the high density of the PTFE. Thus, it is necessary to frequently stir the dispersion in order to avoid settling of the PTFE particles. This is an impractical, expensive method for maintaining the uniformity of the dispersion over time, and renders storage and shipment of the dispersion difficult.
What is still desired, then, is a composition for a charge transport layer of an imaging member that forms an excellent, stable dispersion when particle additives, particularly polytetrafluoroethylene particles, are included in the composition.